1. Field of the Invention
The present invention relates to an axial-flow fan motor to be used for cooling electronic devices, and especially relates to an improvement of bearing parts which receive a thrust load acting on a rotor of the motor.
2. Description of the Prior Art
A constitution of a conventional commutatorless axial-flow fan motor is described referring to FIG. 8, which is a cross-sectional side view showing the constitution of the conventional commutatorless axial-flow fan motor.
As shown in FIG. 8, the conventional axial-flow fan motor comprises a stator assembly 6, a housing 7 and a rotor assembly 11. The stator assembly 6 has insulators 1 which are made of thermoplastic resin, a stator core 2 which is insulated by the insulators 1, driving coils 3 which are wound around arm parts of the stator core 2, a circuit substrate 4 on which a driving circuit is provided and plural connecting pins 5 which are soldered on the circuit substrate 4 for connecting the driving circuit and the driving coils 3.
The housing 7 has a short cylinder shape and has a motor holder 7b which is supported by plural spokes 7a on the center axis part of the cylindrical shape of the housing 7. The housing 7 is formed by resin molding with the motor holder 7b. The motor holder 7b has a tube-shaped bearing holding part 8, which is, for example, made of metal and inserted in the center axis part in the resin molding process of the housing 7. The assembled stator assembly 6 is fit and fixed on the outer face of the bearing holding part 8 by adhesive 9.
Ball bearings 13 and a coil spring 14 are fit in the inner hole of the bearing holding part 8 for bearing a rotor shaft 12 of the rotor assembly 11. The coil spring 14 supplies a pre-load on the ball bearings 13. The rotor assembly 11 has plural blades 10 on its outer periphery.
The above-mentioned conventional axial-flow fan motor, however, is expensive and complex because it uses the ball bearings 13 for bearing the rotor shaft 12 and the bearings 13 need a coil spring 14 for supplying the pre-load.
Furthermore, the axial-flow fan motor is generally used in a high temperature environment. In such a case, base oil of a grease which is used as a lubricant of the ball bearings 13 is gradually evaporated and the viscosity of the lubricant becomes high. As a result, the lubrications between balls and an inner ring and/or balls and an outer ring of the ball bearings 13 are damaged and the ball bearings 13 generate high noise due to the slip of the balls and the rings. In particular, the viscosity of the lubricant becomes acceleratively high with a decrease of the base oil of the lubricant in the ball bearing and the bearing also becomes noisy.
On the other hand, it is considered whether a sleeve type plain bearing (not shown in the figure) made of sintered oilless metal which has many small pores and oil which is pressurized and impregnated can be replaced with the ball bearing or not. In such a case, an initial property of the plain bearing can be maintained even when the lubricant has been lost in some measure in comparison with the case of using the afore-mentioned ball bearing. Therefore, the noise due to the evaporation of the base oil of the lubricant, which is a problem in case of using the ball bearing, is not much of a problem in this case.
The sleeve-type plain bearing, however, has a problem in the way the thrust load is received by the plain bearing. Namely, in such an axial-flow fan motor, a relatively large thrust load is generated by a reaction of an air flow due to the rotation of the fan and is applied to the rotation shaft of the fan. In a constitution that the thrust load is directly received by an end face of the sleeve-type plain bearing and a washer provided on the rotation shaft of the fan (which are not shown in the figure since they are known in the art), a peripheral velocity on the peripheral part of the washer sliding on the end face of the bearing becomes very large and the friction loss damages the life of the bearing. In particular, the end face of the conventional sleeve-type plain bearing is made flat, so that the thrust load is received by the whole of the end face evenly. Thereby, circulation of the oil, contained in the sintered oilless metal due to a pumping operation which is a feature of the sintered oilless bearing, can not be expected. And the pressure durability against the thrust load of the conventional sleeve-type plain bearing is extremely reduced.